Abstract：Carbon nanotube (CNT) fiber is a promising material due to its extensive potential in micro/nanoelectronics, where the thermal performance is of great importance. In this work, a well-developed steady-state electro-Raman-thermal technique is employed and extended to the ambient environment for measuring thermal conductivity of the CNTs fiber. In this technique, two ends of the CNT fiber are attached to heat sinks and a steady electrical current flows in a sample to induce Joule heating. The heat dissipates to the ambient air and goes through the sample to the heat sinks. With combined effects of natural heat convection and heat conduction, a steady temperature profile along the sample can be established. The middle point temperature of the fiber is probed by measuring the local Raman spectrum. It is because the Raman scattering (such as G peak) of CNT fiber is temperature dependent and thus it can be used as a temperature indicator for thermal property measurement. In calibration experiment, the temperature coefficient of the G peak of CNT fiber is first obtained. A modified one-dimensional heat conduction solution involving free convection effect is derived as #br#T(x) =((I2R)/(hLS))(1 -(e√(hS)/(kAc)x)+e-√(hS)/((kAc)x)/(e√(hS)/(kAc)L/2)+e-√(hS)/(kAc)L/2))+ T0. It can be found that the relationship between middle point temperature (T0) and applied Joule heating power (I2R) can be used to extract the thermal conductivity of the material (k) as long as the convection coefficient (h) is available. In this work, the convection coefficient is calculated by the model established by Peirs et al. The thermal conductivity of CNT fiber synthesized from floating catalyst method is measured to be 66.93 W/(m·K)± 11.49 W/(m·K). This value is a little bit larger than that of other CNT fibers synthesized by the acid spun method or the dry-spinning method, which can be explained by the different sample structures induced from different synthesize method. This value is two orders of magnitude smaller than that of individual carbon nanotube, and two orders of magnitude larger than that of CNTs packed bed, showing that heat conduction in CNT based bulk material is determined mainly by a large number of thermal interfaces between CNTs contacts rather than the intrinsic thermal property of CNT.